1. Field of the Invention
This invention relates to cavity resonators and filters. More specifically, this invention relates to tunable resonators, and to filters incorporating tunable resonators.
2. Related Art
Tunable resonators are commonly used in filters and other devices that receive and transmit microwave communication signals. Such resonators are generally used in microwave filters having single or multiple resonators. In operation, the resonators are individually tuned to operate at a specific channel band within a broad range of frequencies and bandwidths. Tuning a conventional resonator may be accomplished by adjusting the position of a dielectric tuning slug with respect to a primary resonator disposed within the tunable resonator. However, practical applications of resonators are constrained by the resonators' tuning range. Although the tunable range of a tunable resonator can be relatively large, the individual channel bands are relatively small, and have a small spacing between adjacent channel bands. Consequently, the resonators must be precisely tuned so that neighboring channels do not interfere with one another during operation.
Tuning resonators over a wide range can be accomplished by adjusting the position of the tuning slug disposed adjacent the primary resonator. Specifically, the tuning slug's position is adjusted until the primary resonator reaches a desired resonant frequency. Tuning a conventional loaded resonator is discussed below with reference to FIG. 1. As shown, a conventional resonator 1 includes a housing 2, a ceramic primary resonator 3, and a ceramic tuning slug 4. To tune the resonator 1, the tuning slug 4 is moved relative to the primary resonator 3. For example, when the tuning slug 4 is moved closer to the primary resonator 3, the resonant frequency decreases. As the tuning slug 4 is moved further from the primary resonator 3, the resonant frequency increases. The tuning slug 4 is relatively large and is directly related to the tuning sensitivity. Additionally, the change in resonant frequency is nonlinear in relation to the change in position of the tuning slug 4. Thus, any movement of the tuning slug 4 will have a relatively large impact on the resonant frequency.
In practice, it is difficult to use a single stage ceramic resonator to tune over a large frequency range, yet still have enough accuracy to precisely tune into individual channels. One way of exerting control during tuning is to use a coarse adjustment mechanism for adjusting the tuning slug over a wide tuning range, and a fine adjustment mechanism for adjusting the tuning slug over a narrow tuning range. Conventional fine adjustment mechanisms often incorporate a finely threaded tuning shaft that is rotated to advance or retreat the tuning slug a relatively small distance within the cavity, and a locking mechanism for locking the tuning slug at a desired location. However, because the tuning slug is relatively large, even small movements of the tuning slug caused by the fine adjustment mechanism may be too coarse for a desired tuning operation. Further, manipulating the locking mechanism to lock the tuning slug may cause the tuning shaft to move the tuning slug a sufficient distance to degrade the tuning performance.
At present, precisely tuning resonators requires laborious manufacturing processes that increase the resonator's manufacturing costs. The costs of filters and other devices employing the resonators are also increased. Thus, a need exists for a tuning mechanism that can easily, reliably and accurately tune a resonator.